1. Field of the Invention
The present invention relates to a Josephson junction device which uses a high temperature oxide superconductor and has small spread in characteristic and a manufacturing method for the same.
2. Description of the Related Art
A high temperature oxide superconductor of a YBaCuO system has a high superconductor critical temperature (Tc) of about 90 K. Therefore, a non-expensive liquid-nitrogen having the boiling point of 77 K can be used as refrigerant, and a small and handy freezer that the low temperature of about 60 K can be easily achieved can be used as a cooling means. Also, the facilities for the maintenance of the low temperature can be simplified.
Conventionally, a superconductive electronic device and circuit with a high speed and low power consumption have been realized using a material such as Nb having a low superconductor critical temperature Tc. For the above reasons, if the device and circuit are possible to be realized using the high temperature superconductor, it contributes greatly to the industry.
A superconductor of a YBaCuO system is a main object for the study and development of application to the electronic device at present. The YBaCuO system high temperature superconductor thin film indicative of a good superconductive characteristic can be obtained through vapor-phase growth under the condition of the high substrate temperature of 600 to 800xc2x0 C. and the high oxygen partial pressure in the order of 100 mTorr. One of the superconductive thin films having especially good characteristic has c-axis orientation in which a c-axis of its crystal structure is perpendicular to the substrate surface. On the other hand, depending on the growth condition, it is possible to form an a- or b-axis orientation film. However, the a- or b-axis orientation film is inferior in the superconductive characteristics such as the superconducting critical temperature Tc and the superconducting critical current value Ic, compared with the c-axis orientation film.
A substrate formed of SrTiO3, MgO, LaAlO3 or NdGaO3 is generally used for the growth of the superconductive thin film in consideration of matching with YBaCuO in lattice constant and thermal expansion coefficient and non-occurrence of solid phase reaction. Also, LaSrAlTaO is used as the substrate, since it has a good lattice matching with YBaCuO and has a low dielectric constant and a relatively large substrate can be formed, as shown in, for example, Journal of Crystal Growth, Vol. 109, pp.447-456, 1991.
The YBaCuO system has a strong anisotropic characteristic like the other high temperature superconductors. The superconductive coherence length is longer in the a- or b-axis direction than in the c-axis direction. The coherence length in the c-axis direction is as very short as about 0.3 nm. Therefore, it is desirable to flow a current in the a- or b-axis direction in wiring sections and Josephson junctions in the superconductor circuit using a high temperature superconductor.
For these reasons, when a high temperature superconductor electronic circuit is manufactured using the high quality c-axis orientation thin film, it is difficult to manufacture the Josephson junction of the high quality in a sandwich type Josephson junction in which it is necessary to flow current in the film thickness direction, i.e., in the c-axis direction of the high temperature superconductor crystal, unlike the conventional superconductor circuit using Nb. In this case, an.edge type Josephson junction device using an edge part of the superconductive thin film is suitable rather than the sandwich type Josephson junction device, as shown in xe2x80x9cIEEE TRANSACTIONS ON MAGNETICS, VOL.27, NO.2, MARCH, 1991, pp.3062-3065xe2x80x9d. Therefore, the study and development of the high temperature superconductor circuit is mainly performed using the edge type Josephson junction device.
In the high temperature superconductor Josephson junction device, the technique is not established for forming a very thin barrier layer as an Al2O3 barrier layer used in the Nb system Josephson junction device having a low critical temperature Tc. Therefore, a non-superconductive oxide film such as a PrBaCuO system film and a superconductive oxide film such as a Co doped YBaCuO system film are deposited on a lower superconductive layer as a barrier layer. The PrBaCuO system crystal has a similar crystal structure to the YBaCuO system crystal and the PrBaCuO system non-superconductor oxide is easy to hetero-epitaxially grow on the YBaCuO layer. Also, the Co doped YBaCuO system superconductive film has a critical temperature Tc lower than YBaCuO superconductive film.
In the above, there is a problem in the spread of junction characteristics such as a superconductive critical current value Ic in a high temperature superconductor Josephson junction device. In the barrier layer forming method using the thin film growth method, it is difficult to grow the thin barrier layer sufficiently uniformly. As a result, the coverage of the lower superconductive layer by the barrier layer is low, so that the spread of the characteristics is large between the junctions. For this reason, the study of the high temperature superconductor Josephson junction device is accomplished by use of an interface control technique without using thin film growth technique, as shown in xe2x80x9cAPPLIED PHYSICS LETTERS, VOL.71, NO.17, OCTOBER, 1997, pp.2526-2528xe2x80x9d. In this method, after the lower superconductive layer is etched to a predetermined shape, a combination of an annealing process in a vacuum and the irradiating process of accelerated ions is performed. Through the combination process, the surface portion of the lower superconductive layer is changed in the crystal structure to have a function as the barrier layer. However, there is the spread of critical current value Ic of (1"sgr"=xc2x18%) even in the 10 samples of the high temperature superconductor Josephson junction device manufactured by the above method.
Also, as the superconductive material for the high temperature superconductor Josephson junction device, a copper oxide system material is used and is practicable in the high temperature region. In this case, however, it is especially difficult to form the barrier layer. Because the coherence length is as short as about 1 to 2 nm in the copper oxide system material, it is necessary that the barrier layer of the Josephson junction also has the film thickness approximately equal to the coherence length. When the film thickness of the barrier layer becomes thicker than the above coherence length, the Josephson current becomes difficult to flow and the quality of the Josephson junction is degraded. As a result, the function sometimes becomes not attained. However, in the conventional method, it is very difficult to form the barrier layer of such a thin film in actual.
In conjunction with the above description, a Josephson junction device and a manufacturing method for the same are disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 3-94486). In this reference, a high temperature oxide superconductor of LnBa2Cu3O7-8 and an insulator of Ln2BaCuO5 which is composed of the same elements as the superconductor are continuously formed in a sputtering apparatus. Then, an annealing process is carried out at a predetermined temperature in an oxygen atmosphere to produce a Josephson junction device.
Also, a method of manufacturing a barrier layer type electronic device is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-317381). In this reference, a YBaCuO oxide superconductor thin film is formed on a substrate. The forming process is carried out by a CVD method, a sputtering method or a vapor deposition method. Next, Fe ions are injected into the oxide superconductor thin film, and the injected ions are diffused in the internal direction of the thin film by a heating process to form a barrier layer. Next, the surface of the barrier layer which has received physical damage through the ion implantation is removed. After that, a YBaCuO oxide superconductor thin film is formed on the barrier layer as a counter electrode of the oxide superconductor thin film.
Also, in Japanese Laid Open Patent Application (JP-A-Heisei 10-173246) is disclosed a high temperature SSNS, an SNS Josephson junction device and a manufacturing method for the same. In this reference, a first superconductive (HTS) layer of the high critical temperature Tc is formed on a substrate, and a first dielectric layer is formed on the HTS layer. The first HTS layer and the dielectric layer have an inclined edge. A 3-layer SNS structure is arranged on the inclined edge to form 4-layer SSNS junction.
Also, the study result of the factor which influences on the resistance of the SNS edge junction using Co-doped YBaCuO as a usual metal layer is described in xe2x80x9cHigh-Resistance SNS Edge Junctionsxe2x80x9d by Brian D. Hunt (6th International Superconductive Electronics Conference 1997).
Therefore, an object of the present invention is to provide a high temperature superconductor Josephson junction device in which a barrier layer having a uniform film thickness and composition can be formed with good reproducibility and with a small spread and which is superior in characteristic.
Another object of the present invention is to provide a Josephson junction device in which a good barrier layer can be formed using a copper oxide system material as a superconductive material, and which is practicable in a higher temperature.
In order to achieve an aspect of the present invention, a method of manufacturing a Josephson junction include:
forming a first superconductive layer on a substrate;
forming an insulating film on the first superconductive layer;
etching the insulating film to have an inclination portion;
etching the first superconductive layer using the etched insulating film as a mask, to have an inclination portion;
forming a barrier layer on a surface of the inclination portion of the first superconductive layer; and
forming a second superconductive layer on the barrier layer and the inclination portion of the insulating layer.
In order to achieve another aspect of the present invention, a method of manufacturing a Josephson junction include:
forming a first superconductive layer on a substrate;
forming an insulating film on the first superconductive layer;
etching the insulating film and the first superconductive layer to have an inclination portion;
forming a barrier layer on a surface of the inclination portion of the first superconductive layer; and
forming a second superconductive layer on the barrier layer and the inclination portion of the insulating layer.
In the above, the first superconductive layer and the second superconductive layer preferably are a copper oxide system superconductive layer. In this case, the copper oxide system superconductive layer may be YBaCuO superconductive layer.
Also, the second superconductive layer may be formed in a same sample chamber without being exposed to an atmosphere, after the inclination portion of the first superconductive layer is formed.
Also, the barrier layer preferably has a film thickness equal to or less than 2 nm. Also, the barrier layer preferably has a perovskite structure. In this case, a material of the barrier layer may have a lattice constant from 0.41 nm to 0.43 nm.
Also, the insulating film may contain at least one element selected from the group consisting of La, Sr, Al, Ca and Ta.
Also, a composition of the barrier layer may be Y1-xBa1CuxOy (x being in a range of 0 to 1). Especially, x may be equal to or less than 0.5 or 0.
Also, the barrier layer preferably includes at least one element selected from the group consisting of La, Sr, Al, Ca and Ta which are supplied from the insulating layer or the substrate.
Also, the first superconductive layer and the insulating layer are formed by a pulsed laser deposition method.
Also, the first superconductive layer may be formed at a first substrate temperature, and the insulating layer may be formed at a second substrate temperature lower than the first substrate temperature.
Also, the etching of a first superconductive layer may include etching the first superconductive layer while inert gas ions are irradiated. In this case, the etching a first superconductive layer may include etching the first superconductive layer while inert gas ions are irradiated and the substrate is rotated.
Also, the etching a first superconductive layer and an insulating layer may include etching the first superconductive layer and the insulating layer while inert gas ions are irradiated. In this case, the etching a first superconductive layer and an insulating layer may include etching the insulating layer and the first superconductive layer while inert gas ions are irradiated and the substrate is rotated.
Also, the insulating layer may be etched while the first superconductive layer is etched such that at least one of the elements of the insulating layer is re-deposited on a surface of the inclination portion of the first superconductive layer.
Also, the forming a barrier layer may include ion-etching a surface of the inclination portion of the insulating film and a surface of the inclination portion of the first superconductive layer. At this time, the ion-etching may include ion-etching the surface of the inclination portion of the insulating film and the surface of the inclination portion of the first superconductive layer, such that at least one of elements of the insulating layer is re-deposited on the surface of the inclination portion of the first superconductive layer. In this case, a surface portion of the inclination portion where the at least one element is re-deposited may form an amorphous layer.
Also, the method may further include heating the substrate in an oxygen atmoshere and crystallizing the surface-portion of the inclination portion where the at least one element is re-deposited. In this case, the crystallizing may be a part of the forming a second superconductive layer.
Also, the method may further include changing a structure and composition of a surface portion of the inclination portion of the first superconductive layer through an ion irradiation damage. In this case, the changing may include crystallizing the surface portion of the inclination portion of the first superconductive layer, while a heating is carried out to the first superconductive layer in an oxygen atmosphere.
In order to achieve still another object of the present invention, a method of manufacturing the Josephson junction include:
forming a first superconductive layer on a substrate;
forming an interlayer insulating layer on the first superconductive layer;
forming a second superconductive layer on the interlayer insulating layer as a barrier layer;
etching the second superconductive layer and the barrier layer to form 2 or more electrodes; and
forming an insulating layer between the 2 or more electrodes.
The first superconductive layer and the second superconductive layer are a copper oxide system superconductive layer. In this case, the copper oxide system superconductive layer is YBaCuO superconductive layer. In this case, the barrier layer has a film thickness equal to or less than 2 nm.
The method may further include carrying out ion irradiation to a surface portion of the first superconductive layer to give a damage to the surface portion of the first superconductive layer, before the barrier layer is formed on the first superconductive layer. In this case, the carrying out ion irradiation may include carrying out the ion irradiation at a temperature of the substrate in a range of room temperature to 700xc2x0 C.
In order to achieve yet still another aspect of the present invention, a Josephson junction include a first superconductive layer formed on a substrate and having an inclination portion, an insulating film formed on the first superconductive layer and having an inclination portion following to the inclination portion of the first superconductive layer, a barrier layer formed on the inclination portion of the first superconductive layer, and having a film thickness equal to or less than 2 nm, and a second superconductive layer formed on the inclination portion of the insulating film and the inclination portion of the barrier layer.
In the above, the first superconductive layer and the second superconductive layer may be a copper oxide system superconductive layer. In this case, the copper oxide system superconductive layer may be YBaCuO superconductive layer.
The barrier layer may have a perovskite structure. In this case, a material of the barrier layer may have a lattice constant from 0.41 nm to 0.43 nm.
Also, the insulating film may contain at least one element selected from the group consisting of La, Sr, Al, Ca and Ta.
Also, a composition of the barrier layer is Y1-xBa1CuxOy (x being in a range of 0 to 1). The value of x may be equal to or less than 0.5 or 0.
In order to achieve another aspect of the present invention, a Josephson junction device includes a first superconductive layer formed on a substrate, 2 or more interlayer insulating films provided on the first superconductive layer to be apart from each other and having a film thickness equal to or less than 2 nm, 2 or more second superconductive layers respectively formed on the interlayer insulating layers.
In the above, the first superconductive layer and the second superconductive layer may be a copper oxide system superconductive layer. The copper oxide system superconductive layer may be YBaCuO superconductive layer.
In order to achieve still another aspect of the present invention, a method of manufacturing a Josephson junction, wherein a material of an insulating layer which is formed on a first superconductive layer is diffused into the first superconductive layer to form a barrier layer on a surface portion of the first superconductive layer, and a second superconductive layer is formed on the barrier layer to form a Josephson junction.